The Moonis a Harsh Witness

January
26, 2007: With binoculars, examine the rugged face
of the Moon. It is pocked with thousands of impact craters
from interplanetary asteroids and comets. Ever wonder why
Earth, a much bigger target, apparently has so few craters?
They're so rare that a pristine example, the Barringer Meteor
Crater in Arizona, is actually a tourist attraction. Did Earth
just get lucky and dodge the heavy artillery?

No,
throughout the history of the solar system, Earth was bombarded
even more than the Moon. But Earth is so geologically active
that earthquakes, volcanoes, and plain old weather are continually
crushing, melting, and reshaping its crust. In short, Earth
is continually destroying evidence of its past, including
evidence of ancient impact craters. Almost all the terrestrial
craters that have been identified—only some 170 at last count—have
been so eroded that essential clues have been erased.

Not
so the Moon. In fact, according to Paul Spudis, a senior planetary
scientist at Johns Hopkins University's Applied Physics Laboratory,
one of NASA's best reasons for returning to the Moon is to
learn more about Earth.

"The
Moon is a witness plate for Earth," declares Spudis,
borrowing an apt term from weapons research. When scientists
want to measure the type, amount, and pattern of damage done
by an explosion, they set up diagnostic "witness plates"
of various materials nearby to register the impact of shrapnel
and radiation.

"Earth
and the Moon occupy the same position in the solar system,"
Spudis explains. "While Earth is a very dynamic planet,
the Moon is a fossil world with no atmosphere. So the Moon
preserves a record of the early history of the solar system
that is no longer readable on Earth."

That's
not just speculation. In the early 1970s, the astronauts on
the last three Apollo missions (15, 16, and 17) returned deep-drill
core samples from three different sites on the Moon. The cores
drilled more than 2 meters into the lunar regolith (the layer
of broken rock and dust covering the Moon).

"The
deepest samples brought up by those drill cores were 2 billion
years old, and largely unchanged since they were laid down,"
Spudis says. And what a surprise recent re-analysis has revealed.
"The lunar regolith traps particles from the solar wind.
And drill cores show that the solar wind had a different chemical
composition 2 billion years ago than it does today. There's
no known explanation for that in solar theory. But that discovery
is crucial for understanding the formation of Earth—and also
the evolution of stars."

Another
big question a return to the witness-plate Moon might help
answer is, What caused the sudden mass extinctions of life
forms on Earth that mark the ends of different geological
eras?

The
most famous is the so-called K-T extinction that wiped out
the dinosaurs 65 million years ago, marking the end of the
Mesozoic Era (the age of reptiles) and the beginning of the
Cenozoic Era (the age of mammals). Much evidence suggests
that an asteroid some 10 km wide slammed into Earth, creating
such catastrophic climate change that photosynthesizing green
plants died, starving more than half of all living beings
worldwide; indeed, ground zero has been identified on Mexico's
Yucatán Peninsula as the Chicxulub Crater, 160 km across.

There's
evidence in the fossil record that such impacts occur periodically,
"once every 26 million years," says Spudis. "Not
everyone agrees, but I think it is pretty convincing."

Why
would this happen? "Some theories are wild!" There
might be a dark, distant companion of the sun that periodically
perturbs comets in the Oort Cloud, and the comets rain down
on Earth. Or perhaps the solar system as a whole is moving
in and out of the plane of the Milky Way galaxy, and this
somehow triggers periodic episodes of bombardment.

Before
we get carried away with theory, however, "we need to
establish whether this really happens," Spudis cautions.
Is Earth truly subjected to periodic bombardment? Again, the
Moon holds the key: Close-up study of the floors of several
hundred lunar craters could confirm or falsify a 26-million
year period. "We have to sample the stuff that got melted
by the shock of impact, and determine the craters' ages."